1. Field of Invention
This invention provides a method and apparatus for working on both sides of a flat workpiece such as a double sided printed circuit board (called hereinafter PCB) while maintaining high relative precision between sides.
2. Description of Prior Art
Various machines for drilling, engraving, and otherwise working flat stock such as a copper clad PCB substrate have been described, built and marketed in the past. Such machines are widely used for the production of PCBs in limited prototype quantifies. They have the advantage of fast production turn-around and require no processing chemicals.
Such machines are typically equipped with a working table and are frequently referred to as flatbed machines. Flatbed machines generally have a machining tool mounted to a spindle motor disposed above and perpendicular to the plane of the table. The tool is caused to traverse is about the bed by the use of X-axis and Y-axis driving apparatuses while the rotating tool is engaged or disengaged to the workpiece as defined by the controlling software. Such a machine is proposed in patent 5,462,512 to Hirioshima (1995). However, this machine has no provision for working a double-sided workpiece.
An alternative to the flatbed machine has been a design employing a rigid U-shaped frame which straddles the flat workpiece. On the end of one arm of the U-frame would be disposed a backup supporting surface. On the other arm would be disposed a spindle motor holding a machining tool perpendicular to the plane of the workpiece and facing the backup supporting surface. The spindle motor would be moveable towards or away from the workpiece surface by use of a z-axis solenoid or motor for the purpose of engaging the working end of the tool into the workpiece according to a depth defined by computer software or by manual settings. When engaged, the tool would apply pressure to the workpiece which, in turn, would be thrust against the backup surface. The workpiece would then be caused to traverse in its plane as the Z-axis machining tool is engaged or disengaged according to the controlling software specifications. Such a machine is proposed in patent 4,786,216 to Kitagawa et al (1988) for a drilling machine. This machine has no provision for working a double-sided workpiece.
Other drilling machines having U-shaped arms have been proposed which have opposing spindle heads but no provision to cause the heads to traverse over the plane of workpiece by a programmable device or calibrated manual positioners. Machines of this description include U.S. Pat. No. 4,215,958 to Jagers (1980) and U.S. Pat. No. 5,152,641 to Overmyer and Peitz, JR. (1962).
Patent 4,967,947 to Sarh (1990) claims, xe2x80x9cin effectxe2x80x9d, to have a C-frame configuration but the C-frame consists of three members having the two arms being independently slidable on the base, which would negate a prime advantage of my patent. This reference also provides for opposing detachable companionate tools, however it is limited to having a fixedly supported workpiece, has no provisions for engraving, and is a much more complex design than my patent.
In order to work both surfaces of a flat a workpiece it has been necessary to first place the workpiece over alignment pins on the machine surface such that the obverse workpiece surface faces the working end of a rotating machine tool. After the obverse surface was completely drilled and engraved, the workpiece would be flipped and again fitted over the alignment pins such that the inverse surface would face the machining tool and the engraving process would then continue to completion.
However, the flipping process created a number of problems. A major problem was an inability to obtain close overall alignment of related machined items on opposing surfaces of the workpiece. An obvious manifestation of this problem would be annular rings engraved on the inverse-side being misaligned with through-holes drilled from the obverse-side surface of a PCB.
Following is a list of some reasons that misalignment can not be eliminated when a workpiece is flipped in order to machine both sides:
The x-axis rails can not be adjusted perfectly perpendicular to the y-axis rails.
The X or Y-axis rails could not be set perfectly parallel or perpendicular to the workpiece pinning groove machined into the bed.
There may be irregularities in the pitch of the lead-screws.
The alignment pins may not be tight or may vibrate loose.
Pinning holes can not be drilled through the workpiece perfectly perpendicular to its plane.
Pins can not be set into the bed/table top perfectly perpendicular to the surface.
The rails can not be manufactured or installed perfectly straight.
Lead-screw wobble affects tool head position.
There could be imperfections in the axis driving motors.
Thermal expansion in lead-screws affects pitch.
None of the above listed reasons would necessarily be objectionable when machining only one surface of a workpiece. However, after being flipped, all the machined imperfections that were created on the workpiece from one table side of the alignment pins would now be associated with the table side opposite the alignment pins. The two machine-sides have totally different and sometimes additive imperfections. Compromise in machine alignment has been necessary in order to achieve overall acceptable double sided accuracy.
Other problems are as follows:
Obtaining acceptable overall alignment during machine manufacture has required many man-hours of highly skilled technical labor.
The larger the workpiece capacity the more difficult it has been to achieve overall machine calibration.
Machines are easily knocked out of alignment during shipping, handling, and normal wear and tear.
The table surface constitutes a considerable cost and weight percentage of a completed machine.
Debris from machining processes tends to settle on the working surface of flatbed machines.
Depth-of-cut is affected when the pressure foot xe2x80x9crides upxe2x80x9d on the debris.
Horizontally positioned flat-bed machines require a considerable amount of table-top space.
Workpiece warpage can cause the machine tool to drag on the work surface during xe2x80x9ctool-upxe2x80x9d moves. This can damage tools and create defects in the work-in-progress.
In addition to pinning the workpiece to the table, taping of the workpiece to the platen is usually required to assure the edges are held down close to the surface and no workpiece movement occurs during engraving.
The workpiece flipping process is time consuming.
On flatbed machines a sheet of backup material equivalent in size to the blank workpiece is required to prevent drilling through the platen surface and to prevent substrate breakout as the drill bit penetrates the bottom surface.
Even though very few holes may have been drilled into the backup material, it can not be reused because of the possibility of a drill bit in the new work stiking a hole from previous work, causing bit breakage or otherwise causing poor quality drilling. Thus, a substantial amount of once-used and substantially unpenetrated backup material is frequently discarded.
Previous machines, having permanently mounted tool heads, require the tool head to be positioned off the edge of the platen surface in order to replace or flip the workpiece or change milling tools and drill bits. This off-board positioning process consumes production time and requires longer rails and lead screws than would be necessary to traverse the bed surface only.
Software and operating procedures are complicated when board flipping is required. A software mirror-image must be created for the flip side which, in turn, must be centered precisely relative to the obverse side. Making multiple small double sided PCB""s on one substrate sheet is further complicated because of the need to offset individual works-in-progress on alternate machine sides. Electrical cable routing to the spindle head of a flatbed design can be complicated by the fact that the head must traverse in both the X and Y axis. Fixed head machines are limited to the head type installed during manufacture of the machine. Additional head options such as a fluid dispensing head must be adapted to the existing head, rendering a more complex overall design.
On flatbed machines, the back side of the work-in-progress cannot be visually monitored, mechanically sensed, or electrically sensed because of the obstacle presented by the bed itself. For example, without first removing the workpiece, it would be difficult to determine whether or not a drill bit is completely penetrating the workpiece.
Boring individual holes from both sides of a workpiece (rather than completely through from one side) in order to prevent material breakout resulted in tool breakage because of flipping misalignment.
Presently, physical design of electronic assemblies requires closer tolerances than ever before and future designs will even be more critical as component dimensions continue to decrease in order to increase component densities and decrease electrical losses. The ability of current apparatus to machine close tolerances into double sided flat work pieces is limited, in large part, because of the need to flip the workpiece in order to access both sides.
Accordingly, several advantages of the present invention are:
to provide an apparatus having the ability to work both sides of a flat workpiece without having to reposition or flip the workpiece within its mounting;
to provide greatly improved relative work precision between opposite workpiece sides;
to enable improved machine alignment stability;
to enable a simplified machine alignment process;
to enable simplified software applications;
to enable simplified operating procedures;
to enable easy access of tool chuck for tool replacement;
to enable user mounting of heads having different functions;
to enable user replacement of defective heads;
to enable use of a small floating backup support surface rather than a precision machined flatbed table, thus:
eliminating associated precision alignment pinning groove in bed;
eliminating need to drill pinning alignment holes through workpiece;
significantly reducing overall weight;
reducing manufacturing material cost, reducing manufacturing labor time;
enabling warped areas of workpiece to make intimate contact with backup support,
enhancing control of depth-of-cut, and reducing disengaged tool drag and;
enabling use of small piece of backup material rather than full sheet as required for flatbed machines. Note: backup material is placed between workpiece and backup support surface or flatbed table to receive tip of drill bit as it completes workpiece penetration.
to enable the workpiece to be positioned vertically or at angle relative to support surface thus:
saving tabletop work space;
enabling workpiece angle to be regulated and hence controlling gravity pressure of substrate against backup surface;
enabling gravity to remove working debris from work surface and;
enabling visual, optical or electromechanical monitoring of work-in-progress from either workpiece side.
It is therefore an object of the present invention to provide a machine capable of precisely, speedily, and completely working both sides of a flat workpiece without having to reposition or flip the workpiece within its mounting.
According to the present invention, the forgoing and other objects can be achieved by providing an apparatus for working on both sides of a flat workpiece, which comprises:
a first support surface having a first axial line thereacross;
a second support surface juxtaposed to and associated with the first support surface and having a second axial line thereacross which lies in a common plane with the first axial line;
a first carrying assembly operably connected to the first support surface in a manner to movably reciprocate along the first axial line and having a first extending arm which is laterally displaced from one side of the common plane and a second extending arm which is laterally displaced from an opposing side of the common plane;
a second carrying assembly operably connected to the second support surface in a manner to movably reciprocate along the second axial line, wherein the second carrying assembly includes means for removably mounting the workpiece in a manner to position the workpiece in the common plane between the arms which extend about the workpiece; and
a tool removably operably connected to one of the extending arms in a manner to permit the tool to perform work on a face of the workpiece.
The method includes movably disposing the flat workpiece in a predetermined plane such that the flat workpiece is movably positionably maintained within the plane; and actuating a pair is of arms about the plane such that the arms are positionably maintained at a predetermined distance from the workpiece, wherein each arm has tool means for working a respective facing side of the workpiece and the tool means are actuatable to act on common coordinate locations of the respective facing sides.